An optical study of M33: I. Morphology of the gas.

A general catalogue of 369 distinct H II regions has been compiled from wide-field photographs obtained with a focal reducer and narrow-band interference filters. Ring-like emission nebulae have been observed as far as 35 min of arc from the nucleus. It is proposed that these rings represent a late stage in the life of expanding ionized regions. General histograms related to the distribution of H II regions, intensities and diameters are plotted. The number surface density of the faint regions is rather constant over the whole galaxy, while for the stronger regions, it is distinctly greater in the central part. A most-probable value of 13 sec of arc is found for the apparent diameter of the H II regions. A detailed spatial comparison is made with radio continuum sources, Wolf-Rayet stars, H I clouds, and clusters of hot stars. Spiral structure is drawn based on the H II and H I observations. A very good correlation is found between H II arms and the distribution of H I clouds. Outer extensions of H emission are related to the spiral structure.